Bacteriophage T7 is a relatively simple virus that has been well characterized physiologically and genetically. The nucleotide sequence of over half of the 40,000 base pairs of T7 DNA is now available, and over 95% of the moleculae has been cloned as specific, well defined fragments. The combination of genetic and physical data has identified 41 known or potential T7 genes, and mutations affecting all but one of them have been found. Very few T7 genes remain to be discovered. At least 10 T7 genes are known to have a role in replication and maturation of T7 DNA. Intermediates produced during replication and maturation are being identified and isolated by sedimentation or gel electrophoresis of extracts of infected cells, and restriction endonucleases are being used to analyze their structures. Kientics of movement of parental and progeny DNA through such forms are being followed after infection by wild-type and mutant phages. Other aspects of DNA metabolism, such as protection of T7 DNA after infection, selective degradation of host DNA, initiation of replication, and mechanism of formation of deletions are also being studied. Interactions between cloned DNA and infecting phages are providing additional information about DNA metabolism after infection.